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The wrong chemical dosage has been used to protect crops against mice as they exhibit lower sensitivity to zinc phosphide than previously thought, new research shows.
Australia’s national science agency, the CSIRO, has recommended a new bait formulation that doubles the amount of zinc phosphide (ZnP) in grain baits used in broadscale agriculture.
The research came in response farmers’ concerns that baits were not being effective, even before the mouse plague of 2021, when mouse numbers surged despite thousands of tonnes of poisons being deployed.
The plague wreaked havoc on agricultural activity and mental health in regional New South Wales and Queensland for more than 10 months, and the effects are still being seen in Western Australia, where mouse numbers remain high.
Steve Henry, a CSIRO research officer specialising in the impact of mice on the grain industry, said: “We’ve listened to farmers concerns, and we then went and did some trials to address those concerns, and we found that their concerns were completely founded.”
Zinc phosphide-coated wheat bait is the only registered in-crop rodenticide in Australia, and has previously been approved for use in a concentration of 25g/kg of active zinc phosphide per wheat grain.
The new research is the first laboratory-based mouse bait efficacy study in Australia since the chemical was registered for agricultural use about 20 years ago.
Henry said the 25g dose can work well in instances where there is not much other food around to distract mice, and they can quickly find two or three grains of the bait, which is enough to get a lethal dose.
If not, he said, “the bait is competing with those other foods for the attention of a mouse”.
“If they have … a dose that doesn’t kill them, then they become bait averse and they stop eating the bait.”
The initial research prompted emergency permits to be issued during the height of the plague, allowing bait producers to double the toxicity of their products from 25g/kg to 50g/kg.
Since then, CSIRO researchers have conducted a series of studies to reassess the sensitivity of mice to zinc phosphide in the laboratory, as well as the effectiveness of a new bait formulation in the field.
Henry said the final study conducted near Parkes in western NSW confirmed that 50g/kg of zinc phosphide to grain bait is required to consistently reduce mouse populations, achieving “more than an 80% reduction in mouse populations more than 90% of the time”.
Rodenticides have been known to have negative effects on non-target species. John Grant, a spokesperson for Wires, Australia’s largest wildlife rescue organisation, said although Wires did not conduct any tests proving animals died from mouse bait poisoning, there were many incidents of mass deaths (upwards of 50 birds) from suspected poisoning during the mouse plague.
Henry said zinc phosphide is not a danger to non-target species, but “it’s very easy to confuse the issues associated with the use of anticoagulants with zinc phosphide”.
“Only farmers are allowed to use zinc phosphide in broadscale agriculture, but the [anticoagulant] toxins that are available for use around towns and around houses are very different to zinc phosphide, and there is a chance of secondary poisoning associated with those baits.”
Zinc phosphide does not cause secondary poisoning because the poison gets used up in the act of damaging the mouse’s major organs, so none is left in the mouse to poison other animals that eat those mice, Henry said.
He said there is little chance of native wildlife being directly poisoned by zinc phosphide because there are very few or no other mammals that live in the relevant paddocks.
When it comes to birds, he says the colour of the bait doesn’t fit within the spectrum of birds’ vision as it’s a dark-coloured bait.
He said farmers also aren’t supposed to spread the bait within a 50m of boundary fences where birds inhabit the trees.
Henry believed any incidents of animal death during the mouse plague were likely caused by different kind of mouse bait to zinc phosphide.
Robert Davis, a senior lecturer in wildlife biology at Edith Cowan University, said “I agree there will be little to no risk of secondary poisoning.”
“However, it is not true to say native species are not at risk from consuming these baits,” Davis said.
“I see them as a risk to any native rodents of which there are many species in Australia and a range of other native marsupials including possums, bandicoots, wallabies and kangaroos.”
“However, the issue of mouse plagues is a tricky one and at the end of the day, this may be the lesser of the evils if used very carefully and sparingly,” Davis said.
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